Electric bolt heater



Dec- 29, 1942- E. F. MILLER ELECTRIC BOLT HEATER Filed July 5, 1941 avg a" 0% ATTORNEY Patented Dec. 29, 1942 ELECTRIC BOLT HEATER Ernest F. Miller, Lansdowne, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 3, 1941, Serial No. 400,991

(Cl. 21S-11),

1 Claim.

My invention relates to an electric heating device ior heating bolts and the like, and it has ior an object to provide apparatus of this character capable of being plugged into an electric circuit of standard voltage, such as 11G-120 volts, and having high heating capacity while at the same time not involving excessively high temperatures oi the resistance heating element or wire.

Heretoi'ore, bolt heaters have been of two types, rst, the type having inlet and outlet leads at one end of the implement and typified by the patent to Hodgkinson, No. 1,839,849, January 5, 1932, and second, the type with one lead connected to the top end of the implement and the other lead connected to the object being heated, the second type being typified by the patent to Bates, No. 2,176,601, October l'l, 1939. The second type is advantageous in that the resistance heating element is directly exposed i'or the ti ansmission oi" radiant heat to the object being heated; however, use involves the complication of a transformer to provide heating current of suitable voltage and amperage. Furthermore, with the second type, diiiiculties occasionally arise because oi poor contact of the lower end of the element with the object being heated and on account of the necessity oi' the resistance heating element having suicient columrt strength to avoid buckling. While the first type does not have these disadvantages, the insulating covering for the resistance heating wire and the outer metallic cover thereof makes it necessary to have a substantial temperature gradient between the wire and the object or bolt to be heated, and, on that account, requiring a rather high temperatuie of the wire or too long a time for heating. Furthermore, the insulating material and sheathing reduces the space available or resistance heating wire. Accordingly, it is a more speciiic object of the invention to provide a construction combining the advantages of radiant heat transmission of the second type with the advantages o the first type and wherein the resistance heating element or wire is so arranged as to give the desired high capacity without the necessity or' using a transiormer.

A more particular object oi the invention is to provide a bolt heating implement comprising a helical winding of resistance heating wire carried by a supporting structure which includes a tubular ceramic core, a metallic core rod therein electrically conductively joined to the lower end of the heating wire, and an insulating foot joined to the lower end of the core rod for supporting the implement on bolt bore bottoms, the core rod -y serving to impart strength and stiffness to the supporting structure such that the implement has the characteristic of a sell-supporting column and the ceramic core having a helical groove of greater depth than the resistance heating wire diameter to provide for exposure of the latter directly to bolt bore walls for the iadiant transmission oi heat to the latter and for iins or lands separating the adjacent convolutions of the heating wire, spacing the latter from bolt bore walis, and serving to guide the implement incident to insertion and removal from bolt bores.

A further object oi' the invention is to provide apparatus of the above character wherein the ns or lands oi the ceramic core separating adjacent convolutions of the resistance heating wire each have a thickness appioximately equal to the wire diameter to provide for a relatively high ratio of wire exposed surface to fin or land exposed surface.

A further object of the invention is to provide an arrangement oi the above character wherein the ce; amic core and the metallic core rod have approximately the same coeliicient oi thermal expansion, whereby minimum dinerential expansion occurs and loosening oi the parts, or possibly dislocation oi the resistance heating wire, are minimized.

These and other objects are effected by the invention as will be apparent from the following description and claim taken in connection with the accompanying drawing, forming a part of this application, in which:

Fig. 1 is a sectional view of a bolt having the improved heater applied thereto; and,

Fig. 2 is an enlarged, longitudinal sectional view of the heater.

In the drawing, there is shown a heating implement at iii, particularly useiul for expanding relatively large bolts to reduce the wienching eiort and sti ain in the bolting by the avoidance of bending and torsional stresses, so that the safety factor has to take into account only the tensile stress requiied in the normal functioning of the bolt to hold parts together in a fluid-tight manner against the force oi relatively high pressures such as occurring in highpressure steam turbines. Such bolts may vary from 11/2 inches to 6 inches or more in diameter, may be as much as 5 feet in length, and are provided with axial bores to receive heating implements, the bore length and diameter depending upon the size of the bolt.

In Fig. 1, parts or flanges il and l2, suoli as steam turbine cylinder anges, are held in nudtight engagement by bolts I3, one bolt being shown. The bolt has an axial bore I4 for the electrical heating implement or device at ID, the latter being a slender cylindrical contrivance for insertion into and removal from the bore.

The heating device or implement includes a core I5, made of heat-resistant and electric insulating ceramic material, having an external helical groove I6 whose adjacent convolutions are separated by the helical lands or ns I'I.

A resistance heating wire I8 of helical formation is located in the groove I6. The groove has such depth that the land or 1in radial depth is well in excess of wire diameter, whereby the lands or fins serve to insulate the resistance heating wire from objects contacted by the implement. Furthermore, in order that the implement may have a relatively large amount of its total exposed surface constituted by the resistance heating wire, the thickness of the land or n is approximately that of the wire diameter, it being necessary to preserve suflicient thickness for the land or n so that the latter may have a reasonable amount of strength.

Preferably, the ceramic core I is of tubular formation; and, within the bore thereof, there is located a metallic core rod 2l having its end portions 22 and 23 extending beyond the ends of the core.

The end portion 22 of the core rod is attached to an end portion 24 of the resistance heating wire by fused metallic connections 25. Preferably, the end portion 24 of the wire is wound about the core rod and the resulting convolution's 26 are welded to the latter. end portions of the core rod and of the resistance heating wire are embodied in a heat-resistant and electric-insulating construction at 2l.

The f'errule, at 2l, telescopes over the joint between the lower end portions of the core rod and of the resistance wire and it cooperates with such joint to provide an insulating supporting construction for the implement. Preferably, the ferrule comprises a sleeve 28 formed by a wrapping of mica and a plug of electric insulating refractory cement disposed within the lower portion o'f the sleeve and cooperating with the latter to form the ferrule bottom. The sleeve is vfastened about the joint by a helical wrapping of binding wire 29 and the refractory cement forming the ferrule bottom abuts the lower end of the core rod and is adapted to abut the bottom of a bore to support the implement.

The insulating supporting construction for the lower end of the implement preferably also supports the ceramic core so that the insulating wire is relieved of load due to the weight of the latter. As shown, this purpose is achieved by having the upper end of the sleeve 2S abut the lower end of the ceramic core to maintain a predetermined spaced relation between the ceramic coie and the joint between the lower end portions of the core rod and of the resistance wire to relieve the resistance wire of loading due to the weight of the ceramic core. In this way, it is assured that stresses to which the resistance wire is subjected are reduced to a minimum, with the result that durability thereof is increased.

The other end portion 23 of the core and the adjacent end portion 3I ci the resistance heating wire are reversely curved in a parallel-like manner and the ends thereof are connected, such as by silver solder at 32 and 33, to bared teLminal portions 34 and 35 of the insulated conductors 36 and 3l. The end portions of the core The joined f rod, the resistance heating wire and the conductors, together with the soldered connections, are embodied in heat-resistant and electric-insulating material, at 39, the latter being so arranged as to insulate the rod and the conductor joined thereto from the wire and the other conductor joined thereto.

Preferably, the body of material, at 39, is comprised by asbestos cord wrapped about the joined elements individually, with these wrappings enveloped by an outer wrapping, asbestos paper being used as a iilling to avoid substantial irregularity in shape of the body, it having heretofore been pointed out that the body should not extend in diameter beyond the diameter of the ceramic core. As the body at 39 ties together the conductors, it will be apparent that either or both conductors or the body may be grasped incident to handling the implement.

Any suitable ceramic material having the desired heat-resistant and electric-insulating properties may be used for the core, and any suitable metal may be employed for the core rod; however, I prefer to use materials for the core and the core rod having substantially the same coefficient of thermal expansion, and, to this end, I nd that a ceramic consisting chiefly of magnesium silicate (MgO-SiOz) is suitable for the core and that a nickel iron alloy containing about 36% nickel (Nilvar) is suitable for the core rod.

From the foregoing, it will be apparent that the heater presents a relatively large amount of heating surface in relation to its total surface, and effectiveness is further promoted by having the resistance heating wire directly exposed to the object to be heated, with the result that a given rate of heat transmission may be maintained with a minimum temperature of the resistance heating wire, or the latter, at a given temperature, may transmit heat at a higher rate. In this connection, it is desirable to heat bolts rapidly in order that they may expand direrentially with respect to the flanges in order that set up may be effected without any particular wrenching effort, and, for this reason, it is essential that a heater shall have adequate capacity.

Turbine flange bolts requiring heating usually have a bore one-fourth the nominal bolt diameter. A satisfactory heating rate is obtained with an output of 50 watts per inch of bolt diameter per inch of length, this rate calling for a temperature of the resistance heating wire of about 1800" F. Therefore, to maintain this unit rate, heaters are provided of diameters and lengths to suit the bolt dimensions. Suitable heating is obtained for normal bolt set up requirements in from 5 to '7 minutes per inch of bolt diameter. Thus, a 11/2 inch bolt would require about ten minutes and a 6 inch bolt from 30 to 40 minutes, depending upon the amount of tightness required and the physical proportions ol the flanges.

Aside from its effectiveness as a heater, the implement possesses many unique features contributing to its effectiveness as well as to its durability and satisfactory operation in service. First of all, the design of the implement, and particularly the disposition of the resistance heating wire, makes it possible to rely on an electric circuit of standard voltage without additional complications, such as special transformers or other equipment. By providing the ceramic core and the metallic core rod made of materials having substantially the same coefficient of expansion, it is assured that the component parts of the implement will not break or separate or loosen to an objectionable extent. The metallic core rod serves as a reinforcement or maintains the straightness of the implement regardless of transverse fracture of the ceramic core. The resistance heating wire and the metallic core rod are effectively insulated from bodies contacted by the implement: the lands or ns being radially deeper than the diameter of the resistance heating wire, they prevent the latter coming in contact with objects engaged by the implement; the covering body at the lower end of the heater not only insulates the lower joined ends of the core rod and the resistance heating wire, but it is so constructed and arranged as to withstand ordinary impacts incident to bottoming in bolt bores; and the insulated body at the top of the implement not only serves to afford the required electrical insulation for the parts covered, but it ties together the conductors such that it may, as well as the conductors, serve as a means by which the implement may be handled. The lands or ns are made as thin as practicable consistent with reasonable strength and durability; and, in this connection, it will be apparent that breakage of the lands may occur to a substantial extent without impairing the effectiveness of the latter to separate the resistance heating wire convolutions and to insulate the latter from objects contacted by the heater. Not only is the implement a self-supporting one carried by an insulating foot adapted to engage a bolt bore bottom, the

self-supporting characteristic being due to the column-like structure to which the metallic core rod contributes strength and stiffness, but the ns or lands separating the convolutions of the resistance heating wire serve to guide the implement with respect to a bolt bore wall incident to its insertion or removal.

While I have shown the invention in but one form, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various changes and modiiications without departing from the spirit thereof, and it is desired, therefore, that only such limitations shall be placed thereupon as are specically set forth in the appended claim.

What I claim is:

In an electrical heating implement for insertion into and removal from bores of metallic bolts incident to expansion of the latter for nut tightening, a core construction including an electrically-conductive metallic core rod, an insulating implement-supporting foot extending below and attached to the lower end of the rod, and an insulating ceramic tube encompassing the f' core rod and having its lower end adjacent to but spaced above the bottom end of the supporting foot; said foot being adapted to contact the bottom of a bolt bore to support the implement and to insulate the lower end of the core rod from the bolt; said core rod being of suicient strength and stiffness to impart to said implement the characteristic of a self-supporting column structure; said ceramic tube having a helical groove opening through its outer cylindrical surface and dened by helical lands or fins which separate the convolutions thereof; a resistance heating wire having a helical winding disposed in the groove so as to t the bottom of the latter and having its lower end electrically conductively joined to the core rod; the radial depth of the helical groove being greater than the resistance heating Wire diameter to provide fins or lands which space the helical winding of such wire inwardly from the cylindrical boundary surface of the ceramic tube to avoid the wire contacting with a bolt bore wall while exposing it for direct radiant transmission of heat to the latter and which serve to guide the implement in a bolt bore incident to insertion and removal; and supply and return leads electrically conductively joined to the resistance heating wire and to the core rod at the upper end of the implement.

ERNEST F. MILLER. 

